The effectiveness of foliar application of humic substances (22.5 % total humic and fulvic acid with 7.1 % water-soluble potassium oxide) (HFA) was evaluated in the Central Rift Valley of Ethiopia on onion (Allium cepa L.) at Melkassa, Dugda, and Merti. Different levels of HFA liquid fertilizer in combination with mineral fertilizers were evaluated for two cropping seasons (2015-2017). The experiment was laid out in randomized complete block design and replicated three times. The result revealed that the application of RNPK (recommended rate of nitrogen, phosphorus, and potassium) only and foliar application of 1.875 Liter (L) and 2.5 L ha-1 HFA with RNPK (46N, 46P2O5 and 51K2O kg ha-1) and half RNPK significantly improved onion marketable yield at Dugda, and at Melkassa foliar application of 1.875 L of HFA with RNPK fertilizers boosted the onion marketable yield significantly. At Merti the onion marketable yield was improved by the foliar application of humic substances up to 16% though statistically not significant. The partial budget analysis also revealed that the application of RNPK and 2.5L ha-1 of HFA and half RNPK gave a maximum marginal rate of return 1108.6 and 2169.9 % respectively with high net benefits at Dugda. At Melkassa also 1.875 L HFA with RNPK gave the maximum marginal rate of return 676.9 % with the highest net benefit. The foliar application of HFA with/without NPK fertilizers benefits onion production. The use of HFA can be a viable alternative within the small-scale onion producers in the study sites.

Keywords: Foliar application, humic and fulvic acid, band application, marginal rate of return, Ethiopia